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Notice to Readers: Interpretation of Provisional Data Presented in Morbidity and Mortality Weekly Report Tables

Periodically, readers of MMWR interpret provisional data about the incidence of notifiable infectious disease presented in MMWR incorrectly as finalized incidence data. This occurred recently in a media report that included a misinterpretation of the provisional acquired immunodeficiency syndrome (AIDS) data reported to the National Notifiable Diseases Surveillance System (NNDSS) and presented in MMWR Table II (1). Although the characteristics of the MMWR morbidity data (provisional versus finalized) are footnoted, this notice reminds readers about the different characteristics of provisional and finalized data.

Provisional NNDSS data are a running count of infectious disease (including AIDS) cases that have not been adjusted for variations in reporting procedures across different states or for delays in reporting. In addition, data in MMWR Table II reflect the date cases were reported to CDC rather than the date persons were actually diagnosed with the notifiable condition. Morbidity data adjusted to reflect the number of notifiable disease cases diagnosed during a given year reflect disease incidence trends more accurately than a comparison of end-of-year provisional data (e.g., week 52 data for a current year) with finalized data. For example, although 46,143 AIDS cases were reported to CDC in 1999, an estimated 41,850 persons were diagnosed with AIDS; duplicates, errors, or persons who had AIDS diagnosed earlier than 1999 accounted for the remainder of cases (2).

To provide timely NNDSS data, state and territorial health departments voluntarily report notifiable disease incidence data to CDC as soon as they become aware of these cases. These provisional data are published each week in MMWR to disseminate the most current national information about infectious diseases to public health officials and health-care providers so they can initiate prevention and control activities. These data include case reports considered "suspect" or "probable" for surveillance purposes in addition to those considered "confirmed"(3). As a result, provisional data are subject to change based on the outcome of further case investigation. As part of the process for finalizing surveillance data, provisional NNDSS data, including AIDS incidence data, also are adjusted for variations in reporting practices and delayed reporting. CDC amends and edits provisional data periodically throughout the year as updates are reported by states. The process for finalizing case counts can take several months to complete after the end of the year. Cumulative (i.e., year-to-date) incidence data from the previous year also are presented in Table II as a crude method to identify aberrations or discrepancies in reported disease incidence data---whether because of disease incidence or reporting artifacts. Because of differences in timing of reports, the source of the data, and the use of different case definitions, NNDSS data published in the weekly edition of MMWR also might differ from data published in other CDC surveillance reports.


  1. CDC. Table II. Provisional cases of selected notifiable diseases, United States, weeks ending December 29, 2001 and December 30, 2000 (52nd week). MMWR 2002;51&52:1170--3.
  2. CDC. Letter dated January 22, 2002 to the Editor, Knight Ridder Washington Bureau, from Harold W. Jaffe, MD, Acting Director of the National Center for HIV, STD, and TB Prevention.
  3. CDC. Case definitions for infectious conditions under public health surveillance. MMWR 1997;46(no. RR-10).

Use of trade names and commercial sources is for identification only and does not imply endorsement by the U.S. Department of Health and Human Services.

References to non-CDC sites on the Internet are provided as a service to MMWR readers and do not constitute or imply endorsement of these organizations or their programs by CDC or the U.S. Department of Health and Human Services. CDC is not responsible for the content of pages found at these sites. URL addresses listed in MMWR were current as of the date of publication.

Disclaimer   All MMWR HTML versions of articles are electronic conversions from ASCII text into HTML. This conversion may have resulted in character translation or format errors in the HTML version. Users should not rely on this HTML document, but are referred to the electronic PDF version and/or the original MMWR paper copy for the official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.

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